In the IEEE 802.11 wireless LAN standard, an infrastructure mode for a wireless terminal to connect to a wireless base station and perform communication via the wireless base station is provided, and furthermore, a power saving function (power save mode) for suppressing power consumption of the wireless terminal in the infrastructure mode is defined (IEEE Std. 802.11-2007, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”). This is a function to realize power saving not on the wireless base station side but on the wireless terminal side, and the function is intended to suppress power consumption by the terminal causing a wireless communication module to transition to a sleep state in synchronization with a beacon signal transmitted from the base station cyclically. Since the wireless terminal which has transitioned to the sleep state cannot receive data, the wireless base station temporarily buffers the data and gives a notification to that effect to the wireless terminal by including a TIM (Traffic Indication Map) in a beacon signal. The wireless terminal which has received the notification can receive the data from the wireless base station by performing polling to the wireless base station.
On the other hand, a method for realizing power saving of a wireless base station in the infrastructure mode is shown in JP-A 2004-336401 (Kokai). In general, a wireless base station in the infrastructure mode is required to always wait for data transmission from a wireless terminal though it is not known when the data transmission occurs. Therefore, there is a problem that the wireless base station cannot transition to a sleep state. JP-A2004-336401 proposes a method in which the wireless base station transitions to the sleep state by transmitting a CTS (Clear To Send) frame inhibiting transmission by a wireless terminal to the wireless terminal before transitioning to the sleep state.
In the related-art technique described above, each of the power-saving function on the wireless terminal side and the power saving function on the wireless base station side are shown. However, a method for causing both power saving functions to operate at the same time is not described. Recently, mobile terminals, such a notebook PC (Personal Computer) and a smartphone, are being multi-interfaced (3G, WiMAX, Wi-Fi and the like), and it is expected that there will be a demand for accessing the Internet via a mobile terminal by setting the mobile terminal as a wireless base station and connecting peripheral gadget equipment (a game machine, a music reproduction machine and the like). In such a configuration, both of the wireless base station and the wireless terminal are possibly mobile terminals the battery of which is limited. Therefore, it is important to concurrently realize power saving on both of a wireless base station and a wireless terminal constituting an infrastructure.
As described before, the power saving function of a wireless terminal provided in IEEE Std. 802.11-2007, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications” performs data transmission and receiving at a timing when the wireless terminal releases the sleep state. Therefore, in view of power saving of the wireless terminal, it is desirable to shorten the period during which data transmission and receiving is enabled and keep the period of the sleep state long. On the other hand, a system of inhibiting data transmission by a wireless terminal is adopted in JP-A2004-336401, and a CTS frame instructing inhibition is transmitted with the use of a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) mechanism. Therefore, since the opportunity for a wireless terminal to transmit data and the opportunity for a wireless base station to transmit a CTS frame are equally maintained, there is a possibility that the wireless terminal receives a CTS frame before transmitting data. In this case, the wireless terminal cannot transition to a sleep state immediately, and a problem of reduction of the power saving effect is caused. Furthermore, JP-A2004-336401 proposes a control method of giving priority to the opportunity for a wireless terminal to transmit data by controlling a back-off time in a contention window. However, it is on the first transmission opportunity that this priority control acts, and a wireless terminal which transmits multiple data sequentially cannot avoid receiving a CTS frame before completing the transmission of the multiple data and cannot transition to a sleep state immediately. Therefore, a problem of reduction of the power saving effect is caused similarly.